This invention relates to a phase-locked loop (PLL) type synthesizer with modulation function which is suitable for modulating a carrier signal by a digital signal or a voice signal. Such a synthesizer is particularly useful in a radio communication device.
A synthesizer of the type described is disclosed in U.S. Pat. No. 4,052,672 issued to Enderby et al and assigned to Motorola, Inc., and comprises a phase-locked loop circuit comprising a phase comparator, a loop filter, a voltage controlled oscillator having a gain Kv, and a circuit for feeding back a frequency signal produced from the voltage controlled oscillator to the phase comparator. The feedback operation is carried out through a frequency divider having a frequency division ratio (N). For modulation, a digital signal is supplied to a specific section of the phase-locked loop circuit as a first modulation signal through a first adding circuit. A voice signal is supplied to another section of the phase-locked loop circuit as a second modulation signal through a second adding circuit. Such a synthesizer is desirable to have a constant modulation sensitivity and a flat frequency characteristic over a wide range. The modulation sensitivity may be called audio sensitivity. The modulation sensitivity represents an input level of the modulation signal necessary for giving a predetermined reference variation to a modulated signal.
Although the synthesizer of the type described has a flat frequency characteristic over a full frequency band of the modulation signal, it tends to be influenced by the output frequency, namely, the carrier frequency, of the voltage controlled oscillator. Namely, if the voltage controlled oscillator has a wide frequency band, the frequency division ratio N of the frequency divider (or the gain Kv of the voltage controlled oscillator) varies in dependence on the carrier frequency of the voltage controlled oscillator. This means that the modulation sensitivity fluctuates in dependence on the output frequency of the voltage controlled oscillator.
In order to obtain a constant modulation sensitivity, a phase comparator of a variable gain type has been used for another type of synthesizer. Such a synthesizer is disclosed in U.S. Pat. No. 4,313,209 issued to Drucker and assigned to John Fluke Mfg. Co., Inc. In that synthesizer, a gain of the phase comparator is controlled in proportion to the frequency division ratio N and in inverse proportion to the gain Kv. Simultaneously, the input level of the modulation signal is controlled in inverse proportion to the gain Kv. Although the synthesizer has a constant modulation sensitivity, it needs a variable gain type phase comparator which is expensive.